To Re-cap the classic series used codes for each story which started with A (for An Unearthly Child) and ran to 7Q (Ghost Light). ‘I’ was never used, ‘O’ and ‘U’ sparingly, but as for the rest… And the Tour followed, even extended this into nu-Who all the way forward to Whittaker.
Previous stops in this ramshackle series included stops at ‘B‘ and ‘R‘ and now the laws of consonant-al shift move to ‘X.’ The first stop The Ark is a unique pair of linked 2-parters, one of the storytelling experiments Doctor Who could afford when seasons were 40+ episodes a year, Death to the Daleks was as dull as dishwater, but Image of the Fendahl was the last gasp of the gothic Hinchcliffe/Holmes run. The Mark of the Rani introduced the Rani, The Woman Who Fell to Earth introduced Jodie Whittaker, and The Satan Pit put the hell in hell-o.
Pretty good company wouldn’t you say?